Reactance filter having an improved edge steepness

ABSTRACT

A reactance filter, which is constructed from BAW resonators, has at least one basic element that has a first resonator in a first branch and a second resonator in a second branch. In one branch, there is situated a resonator having a greater ratio of dynamic to static capacitance than in the second branch, so that a filter is obtained having a resulting passband in which one edge is set steeper than the other edge. The selection of the edge that is to be set steeply takes place through the allocation of the first branch to the serial or to the parallel branch.

BACKGROUND OF THE INVENTION

The present invention relates to bulk acoustic wave filters (also knownas BAW filters) that are constructed according to the reactance filterprinciple.

From an article by K. M. Lakin et al. in Microwave Symposium Digest,IEEE MTT-S International 1995, pp. 883–886, it is known to constructreactance filters from BAW resonators. Here, these resonators are usedas impedance elements, and are for example wired or connected to formladder-type or lattice filters. This type of wiring for the manufactureof filters is also known as branching technology.

According to FIG. 1 a, in its simplest specific embodiment a BAWresonator R is made up of a thin film P of a piezoelectric material,which is provided with an electrode E1, E2 on its upper and lower siderespectively. Ideally, this structure is surrounded by air on bothelectrode sides. When an electrical voltage is applied to theelectrodes, an electrical field acts on the piezoelectric layer, withthe result that the piezoelectric material converts a part of theelectrical energy into mechanical energy in the form of acoustic waves.These waves propagate parallel to the field direction, as what are knownas bulk waves, and are reflected at the electrode/air boundary surfaces.At a particular frequency f_(r), which is dependent on the thickness ofthe piezoelectric layer or on the thickness of the bulk resonator, theresonator exhibits a resonance, and thus behaves like an electricalresonator.

In the equivalent circuit diagram according to FIG. 1 b, the BAWresonator R is made up of a series resonance circuit of dynamicinductance L1, dynamic capacitance C1, and dynamic resistance R1, aswell as a static capacitance C0, connected thereto in parallel. Theseries resonance circuit reproduces the behavior of the resonator in theresonance case, i.e., in the range of resonance frequency f_(r). Staticcapacitance C0 reproduces the behavior in the range f<<f_(r) andf_(r)>>f. Dynamic capacitance C1 is thereby proportional to the staticcapacitance C0 of the BAW resonator.C1˜C0.  (1.1)For the resonance frequency f_(r) and the anti-resonance frequency f_(a)of a BAW resonator, the following hold:

$\begin{matrix}{f_{r} = {\frac{1}{2\pi\sqrt{{L1} \cdot {C1}}}\mspace{14mu}{and}}} & (1.2) \\{f_{a} = {f_{r}{\sqrt{1 + \frac{C1}{C0}}.}}} & (1.3)\end{matrix}$

According to FIG. 7, a reactance filter is made up of at least one basicelement that has a serially connected resonator R2 having a resonancefrequency f_(rs) and an associated anti-resonance frequency f_(as), andthat has a second resonator R1 that is connected parallel to a secondterminal, in particular parallel to ground, having a resonance frequencyf_(rp) and an associated anti-resonance frequency f_(ap). In order toproduce a filter having a bandpass characteristic and a center frequencyf₀, the following relation holds for the two resonators in the serial orin the parallel branch:f_(ap)≈f_(rs)≈f₀  (1.4).

FIG. 16 a shows the curve of the impedance Zs of the serial resonatorand of the admittance Yp of the parallel resonator, plotted over thefrequency f. FIG. 16 b shows the passband response of a filter made upof a basic element, whose resonance frequencies are selected as in FIG.16 a. FIG. 7 shows a basic element that is to be regarded in principleas a two-port network having terminals 3-1 or 3-2 as a port 1 and havingterminals 3-3 or 3-4 as a port 2. At the same time, terminal 3-1 is theinput and terminal 3-3 is the output of the series resonator. The inputof the parallel resonator is connected with terminal 3-1. Terminals 3-2and 3-4 represent the reference ground, given asymmetrical operation.The output 3-5 of parallel resonator R1, which faces the referenceground, is designated in the following as the output or ground side ofthe parallel resonator. The inductance L_(ser), which is situatedbetween the output side of the parallel resonator and the referenceground, reflects the connection to the housing ground in the realconstruction.

The selection level of a reactance filter constructed from BAWresonators is determined on the one hand by the ratio C0 _(p)/C0 _(s) ofthe static capacitance COP in the parallel branch and the staticcapacitance C0 _(s) in the series branch, and on the other hand by thenumber of basic elements that are cascaded, i.e., connected in serieswith one another.

A plurality of basic elements can be wired together in matched fashion,whereby the structure of each of the second adjacent basic elements ismirrored. The output impedance of the first basic element (7-1 in FIG.2, or 8-1 in FIG. 3) is then equal to the input impedance of the secondbasic element (7-2 in FIG. 2 or 8-2 in FIG. 3), so that only minimallosses are produced by mismatching. Many structures are known for thewiring of a plurality of basic elements. Some examples are shown inFIGS. 4 and 5.

Resonators of the same type (series resonators or parallel resonators)that are situated immediately one after the other in a circuit of areactance filter can also be respectively combined to form a resonator,whereby the overall capacitive effect of the combined resonator remainsconstant.

From equations (1.2) to (1.4), it can be seen that both the maximumachievable bandwidth and also the steepness of the edges of such areactance filter depend on the difference of the resonance andanti-resonance frequencies of the individual resonators. This differencein turn results from the ratio of dynamic capacitance C1 and staticcapacitance C0. Because these capacitances are proportional to oneanother, the capacitance ratio C1/C0 does not change when one of thesecapacitances is altered. For example, C0 could be varied by changing thesize of the resonator. As a rule, all resonators of a reactance filterhave the same relative bandwidth(fa−fr)/f0.

Curve 1 in FIG. 6 shows the passband response of a reactance filter thatis constructed from uniform BAW resonators, with each resonator having arelatively large ratio of dynamic to static capacitance. The individualresonators thus have a relatively large bandwidth. Curve 2 is thepassband curve of a corresponding reactance filter made up of resonatorshaving a small ratio of dynamic to static capacitance, and thus arelatively low bandwidth of the individual resonators. In the first case(curve 1), a bandpass filter is obtained having a high bandwidth and alow edge steepness, while in the second case (curve 2) a bandpass filteris obtained having a low bandwidth and a high edge steepness.

If it is now attempted, in such a steep-edged filter, to increase thebandwidth to the level of the filter having the larger capacitance ratioby increasing the center frequencies of series resonators and/orreducing the center frequency of the parallel resonators, a strongmismatching results in the center of the passband, because nowf_(ap)<<f_(rs). Condition (1.4) is thus no longer fulfilled. For thisreason, the losses in the center of the passband also increase morestrongly.

Another possibility for broadening a steep-edged filter consists in areduction of the ratio (C0 _(p)/C0 _(s)) of the static capacitance C0_(p) in the parallel branch and the static capacitance C0 _(s) in theseries branch. In this way, the bandwidth can be enlarged to a certainextent without losing the self-matching and the small losses connectedtherewith. However, with this measure the selection level of the BAWreactance filter is strongly reduced, so that the filter can no longermeet possible selection demands, and can for example no longersufficiently attenuate undesired frequencies.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a reactancefilter constructed from BAW resonators that has an improved edgesteepness with sufficient bandwidth, without having to accept anadditional matching or a reduction of the selection level for thispurpose.

This object is achieved according to the present invention by areactance filter constructed from resonators of the BAW type, the filtercomprises at least one basic element having a first resonator in a firstbranch and a second resonator in a second branch, one of the branchesbeing a serial branch and the other branch being a parallel branch, eachresonator having a specific ratio V_(C)═C1/C0 of a dynamic to staticcapacitance, the ratio V_(C) for the resonator of the second branch isset smaller than the ratio for the first branch.

The present invention exploits the fact that for the RF filters in manymobile radiotelephone systems, high demands are placed only on the banddemarcation from the corresponding other duplex band. That is, as a rulean RF filter requires a steep edge only on the side of the passbandfacing the other duplex band. In the currently standard mobileradiotelephone systems based on GSM, CDMA, AMPS, or TDMA, in the case ofa receive filter this is the left edge, while in the case of a transmitfilter it is the right edge.

The present invention makes use of this fact, and indicates a reactancefilter that is constructed from resonators of the BAW type. It comprisesat least one basic element having a first resonator in a first branchand having a second resonator in a second branch that are connectedparallel to one another, one of the branches being the serial branchwhile the other branch is a parallel branch. Each of the resonators hasa specific ratio V_(C) of dynamic capacitance C1 to static capacitanceC0:V _(C) =C1/C0whereby according to the present invention the ratio V_(C) for theresonator of a first branch is set lower than for the resonator of thesecond branch. Dependent on the branch in which the ratio V_(C) has beenset lower, the reactance filter according to the present invention has apassband response having an improved edge steepness for one edge. Theother edge, as well as the remaining resonator and filtercharacteristics, remain unaffected by this change. If, for example, in aresonator in the serial branch the ratio V_(C) is reduced in relation tothe corresponding ratio V_(C) in the resonator of the parallel branch,the right edge of the passband is set steeper, i.e., the edge thatdemarcates the passband from higher frequencies. Analogously, in areactance filter in which the resonator in the parallel branch has asmaller ratio V_(C) than does the resonator in the serial branch, apassband is obtained having a left edge that is set steeper. Because, ina basic element of a reactance filter, the resonance and anti-resonancefrequencies of the parallel resonator are lower than the correspondingfrequencies of the serial resonator, for example the right edge of thepassband is determined by the characteristics of the serial resonator.The steepness of the right edge can be seen in the speed with which theimpedance curve of the serial resonator climbs from the resonancefrequency to the anti-resonance frequency. A steeper impedance increasein a (serial) resonator is obtained when the distance between theresonance and the anti-resonance frequency of the resonator is reduced.Because, conversely, the steepness of the left edge is determinedessentially by the parallel resonator or by the resonator in theparallel branch, a steeper setting of the left edge is achieved througha reduction of the distance between the resonance and anti-resonancefrequency of the parallel resonator.

Because as a rule a real reactance filter is obtained by wiring aplurality of basic elements together, a reactance filter standardlycomprises a plurality of serial resonators and a plurality of parallelresonators. A reactance filter according to the present invention isthen already obtained when the cited modifications have been carried outin a single resonator of one type (serial or parallel). A furtherimproved, even steeper edge is obtained if a plurality of resonators ofone type, preferably all resonators of one type, have a smaller distancebetween the resonance and anti-resonance frequency. Through the givendependence of the corresponding quantities on one another, this distanceincreases with the cited ratio V_(C) of the dynamic to the staticcapacitance. A resonator having such a reduced distance is designated anarrowband resonator. A resonator having a correspondingly largerdistance of the resonance and anti-resonance frequency is designated abroadband resonator.

Independent of the use of at least one narrowband resonator for a firstbranch, the broadbandedness of the filter, i.e., the width of thepassband, is achieved in that broadband resonators are used in thesecond branch.

With a filter according to the present invention, having for example animproved, steeper right edge, a higher selection is achieved atfrequencies that are somewhat higher than the highest frequency of thepassband. This is for example advantageous in a filter that is used incurrent GSM-based or CDMA-based mobile radiotelephone systems as thefilter in the transmission path, which must provide a high degree ofsuppression of the receive band.

Conversely, a filter according to the present invention, having forexample an improved, steeper left edge, is achieved through narrowbandparallel resonators resulting in a high selection at frequencies thatare somewhat lower than the lowest frequency of the passband of thefilter. Such filters are preferably used as filters in the receive pathof current GSM-based or CDMA-based mobile radiotelephone systems, whichmust provide a high degree of suppression of the transmit band.

Via the known connection, according to the following equation, of theeffective coupling coefficient K² _(eff) with the position of theresonance and anti-resonance frequency:

$\begin{matrix}{K_{eff}^{2} = {\left( {\pi/2} \right)^{2} \times {\frac{{fa} - {fr}}{fa}.}}} & (1.5)\end{matrix}$It results that a narrowband resonator can also be achieved through thedirect influencing of the effective coupling coefficient K² _(eff). Anarrowband resonator can be realized on a suitable piezoelectricmaterial having a lower effective electromechanical couplingcoefficient. This effective electromechanical coupling coefficient is inturn obtained from the sum of the effective couplings of all modescapable of propagation in a piezoelectric material.

Because as a rule a real filter uses only one mode, while in contrastthe center frequencies of the remaining modes are at a sufficientdistance from the passband, the effective coupling (for the mode used)can be determined from the equivalent circuit diagram of a BAW resonatoraccording to the following equation:

$\begin{matrix}{k_{eff}^{2} \approx {\frac{C1}{C_{1} + C_{0}}.}} & (1.6)\end{matrix}$From this formula there results the dependence of the narrowbandednessof a resonator on the ratio of the dynamic to static capacitance of aresonator, or, precisely stated, on the ratio of dynamic to staticcapacitance of the relevant oscillation mode, or the oscillation modethat is in use, of the resonator. From this consideration it resultsthat a BAW resonator having a smaller ratio V_(C) has a lower effectivecoupling k² _(eff). If for the construction of a resonator apiezomaterial is used having a small coupling coefficient, and thus loweffective coupling, a resonator is obtained having a small distancebetween the resonance frequency and the anti-resonance frequency. Giventhe use of a higher-coupling piezomaterial, a resonator is obtainedhaving a greater distance between the resonance frequency and theanti-resonance frequency.

A reactance filter according to the present invention therefore has, forexample, resonators having a higher-coupling piezomaterial in the seriesbranch of the reactance filter, and, in contrast, resonators havinglower-coupling piezomaterial in the parallel branch of the same filter.Such a filter then has a high steepness of the left edge. At the sametime, the reactance filter according to the present invention has a highbandwidth, ensured by the relatively great distance between theresonance frequencies and the anti-resonance frequency in the seriesresonators.

Moreover, in a reactance filter having BAW resonators the effectivecoupling can be reduced, when an additional layer which is made of anon-piezoelectric material is inserted between the two electrodes of aBAW resonator. Here the coupling coefficient is reduced by the ratio ofthe layer thickness of the non-piezoelectric material to the overalllayer thickness of the resonator. In any case, with such a layer, areduction of the effective coupling is obtained that for the filter orthe resonator is equivalent to a reduction of the ratio V_(C), and thusis also equivalent to a reduction of the distance between the resonanceand the anti-resonance frequency.

Another possibility for affecting the effective coupling consists in theselection of suitable electrode materials for the BAW resonators. A highelectromechanical coupling is achieved using an electrode material thateffects a high mechanical impedance of the electrode for the mode used.An electrode material that increases the effective coupling (for therelevant or employed mode in the resonator) is obtained dependent on theposition of the electrode metal in the periodic table of the elements,or is determined as an empirical value. A reactance filter according tothe present invention therefore has for example resonators that in afirst branch use an electrode material that differs from the electrodematerial of the resonators in the second branch. For example, throughthe use of a heavy electrode material, such as for example tungsten, theeffective coupling is increased, whereby a resonator is obtained that ismore broadbanded in comparison with a resonator having aluminumelectrodes. A reactance filter having resonators having tungstenelectrodes in a first branch and having resonators having aluminumelectrodes in a second branch accordingly has a more narrowbandresonator in the second branch. If the second branch is a parallelbranch, the left edge of the passband of the reactance filter isimproved. If the correspondingly narrowbanded resonator is used in theserial branch, the right edge is improved in the reactance filter.

A BAW filter is preferably surrounded by air on both sides of theelectrodes. For this purpose, in the technical realization two supportpoints situated at a large distance from one another are provided for anelectrode layer; here one speaks of what are called bridge resonators.In these bridge resonators, the acoustic wave is reflected on both sidesof the resonator at the solid element/air transition. However, it isalso possible to construct a BAW resonator in such a way that one of theelectrodes is situated with its entire surface on a substrate. Thereflection of the acoustic wave can then be ensured using an acousticmirror, which can for example be realized by two layers having differentacoustic impedance, with each layer having a layer thickness of λ/4 inrelation to the wavelength λ of the acoustic wave inside the layermaterial. The repeated reflections at the transitions of the two layershaving sharply differing acoustic impedances then results in theextinguishing of wave portions reflected at different boundary surfaces,which in turn means a high degree of reflection for the mirror.

However, in the use of a resonator having an acoustic mirror a part ofthe mechanical energy of the resonator is located outside theelectrodes. Within the layer sequence electrode/piezomaterial/electrode,the ratio of the electrical to the mechanical energy therefore changes,and thus the effective coupling, measured according to the followingequation, also changes:

$\begin{matrix}{k_{eff}^{2} \approx {\frac{u_{E}}{u_{E} + u_{M}}.}} & (1.7)\end{matrix}$

Here u_(E) denotes the electrical energy density, and u_(M) denotes themechanical energy density. From this equation, it clearly follows thatthe effective coupling of a resonator having an acoustic mirror isreduced in relation to a bridge resonator by the amount u_(M). Thismeans that resonators having an acoustic mirror have a lower effectivecoupling, and thus a smaller distance between the resonance andanti-resonance frequency, than do bridge resonators. A reactance filteraccording to the present invention therefore has bridge resonators forexample in the parallel branch, and in contrast has in the serial branchresonators having an acoustic mirror, whereby a bandpass response isobtained having a steeper passband in the right edge.

It is also possible to influence the effective coupling k² _(eff) of aresonator through the use of different acoustic mirrors. This can takeplace through the use of mirror layers having different thicknesses, orthrough the use of mirror layers having different material. A reactancefilter according to the present invention is then distinguished byresonators using, in a first branch, acoustic mirrors that are at leastpartly different than those used in a second branch.

In the following, the present invention is explained in more detail onthe basis of exemplary embodiments and the associated Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a to 1 c show a BAW resonator with FIG. 1 a being a schematiccross-sectional view, FIG. 1 b being an equivalent circuit diagramthereof, and FIG. 1 c illustrating a substitute symbol used for aresonator.

FIGS. 2 and 3 show circuit diagrams for two possibilities for the wiringof two basic elements to form a filter.

FIG. 4 shows a circuit diagram for a reactance filter having three basicelements.

FIG. 5 shows a circuit diagram for a reactance filter having four basicelements.

FIG. 6 is a graph showing the attenuation curves for a broadband filterand for a narrowband filter.

FIG. 7 shows a circuit diagram for a basic element of a reactancefilter, constructed from BAW resonators.

FIG. 8 shows a circuit diagram for a simplified filter structure havingthree basic elements.

FIG. 9 shows a circuit diagram for the same filter having a simplifiedstructure.

FIGS. 10 and 11 are graphs showing the bandpass responses of reactancefilters according to the present invention.

FIG. 12 is a graph showing the impedance curves of resonators havingdifferent electrode materials.

FIG. 13 is a cross-sectional view of a resonator having an additionaldielectric layer.

FIG. 14 is a cross-sectional view of a bridge resonator in schematiccross-section.

FIG. 15 is a cross-sectional view of a BAW resonator having an acousticmirror.

FIGS. 16 a and 16 b are graphs with FIG. 16 a showing superimposedadmittance and impedance curves for individual resonators, and

FIG. 16 b showing the attenuation characteristic of a reactance filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Exemplary Embodiment

Resonators constructed as in FIG. 1 a are wired together to form areactance filter (see FIG. 1 b). Each resonator comprises a firstelectrode layer E1, a piezoelectric layer P and a second electrode layerE2 (FIG. 1 a). In FIG. 1 c, the symbol standardly used for resonators isshown.

FIG. 7 shows a basic element constructed from a first resonator R1 in aparallel branch and a second resonator R2 in a serial branch. Terminals3-1 and 3-2 form the input of the filter, and terminals 3-3 and 3-4 formthe output of the filter. The parallel branch, or resonator R1 in theparallel branch, is connected with terminals 3-2 or 3-4 via a seriesinductance L_(ser), formed from the sum of the inductances of theconnection to the housing ground. According to the present invention, inthis exemplary embodiment resonator R1 is formed with a piezoelectriclayer P made of zinc oxide, which has an electromechanical couplingconstant K² _(eff) 1, and resonator R2 is formed with a piezoelectriclayer P made of aluminum nitride, which has a piezoelectric couplingconstant K² _(eff) 2, such that K² _(eff) 1>K² _(eff) 2. Via therespective thickness of the piezoelectric layer, or the thickness of theoverall resonator, the resonance frequencies, and thus also theanti-resonance frequencies, of the two resonators R1 and R2 are set suchthat the resonance frequency of R2 is approximately equal to theanti-resonance frequency of R1.

Passband curve 2 in FIG. 10 shows the attenuation characteristic of areactance filter according to this exemplary embodiment of the presentinvention, presented in comparison with passband curve 1 of aconventional reactance filter, in which both resonators use zinc oxidefor the piezoelectric layer P of the resonators. It can be seen that theright edge of curve 2 is set significantly steeper than that of theknown filter. The bandwidth of the overall filter is reduced onlyinsignificantly.

Second Exemplary Embodiment

Again, a reactance filter is constructed having a basic element wiredaccording to FIG. 2, and both resonators are fashioned as in FIG. 1. Incontrast to the first exemplary embodiment, both resonators do comprisethe same piezoelectric material for the layer P, but differ in theelectrode material used for electrodes E1 and E2. While aluminum is usedfor resonators R1, tungsten is used as the electrode material forresonators R2. Because k² _(eff) 2>k² _(eff) 1 holds for the effectivecoupling k² _(eff), as a result a reactance filter is obtained whosepassband curve 2 is shown in FIG. 11. It can be seen that curve 2 of thefilter according to the present invention has a left edge that is setsignificantly steeper than the left edge of curve 1, which shows thepassband response of a known reactance filter in which the sameelectrode material (tungsten) was used for both resonators.

FIG. 12 shows the influence of the electrode material on the impedancecharacteristic of a resonator. Curves 3 and 4 show the impedancecharacteristic of resonators fashioned according to FIG. 1, wherebycurve 3 shows the impedance of a resonator having aluminum electrodeswhile curve 4 shows the impedance characteristic of a resonator havingtungsten electrodes. It can be seen that the greater effective couplingof tungsten electrodes according to curve 4 results in a greaterdistance of the resonance frequency from the anti-resonance frequency.

Third Exemplary Embodiment

A resonator is formed according to FIG. 13. This resonator comprises,between a first electrode E1 and a second electrode E2, made for exampleof aluminum, a piezoelectric layer P made for example of aluminumnitride, as well as a dielectric layer D, made for example of siliconoxide. If the layer portion of the silicon oxide layer is 16%, thecoupling coefficient k² _(eff) decreases from a value of 0.0645,determined in a resonator according to FIG. 1 having aluminum nitride asa piezoelectric layer, to a value of 0.057 for the resonator accordingto the present invention, as shown in FIG. 13. This latter resonatortherefore has a smaller distance between the resonance and theanti-resonance frequency, and can be used in combination withconventional resonators (see FIG. 1), whereby in the reactance filter(for example according to FIG. 7) the serial and parallel resonators areformed differently, i.e., respectively according to FIG. 1 or FIG. 13.

Fourth Exemplary Embodiment

FIG. 14 shows a BAW resonator formed as a bridge resonator. Thisresonator has a basic element corresponding to FIG. 1, but is howeverconnected with a substrate S via two socket structures F. Because thepredominant part of the lower electrode E1 of the resonator has air as aboundary surface, this bridge resonator behaves approximately as aresonator that can oscillate completely freely. At the two boundarysurfaces E1/air or E2/air, total reflection of the acoustic wave therebytakes place.

FIG. 15 shows a resonator that is situated on a substrate S with the aidof an acoustic mirror AS.

A reactance filter according to the present invention is nowmanufactured from at least one basic element (for example according toFIG. 7), whereby in a first branch, resonators having a bridgeconstruction are used, while, in contrast, in a second branch,resonators having an acoustic mirror are used. Because the effectivecoupling coefficient for resonators according to FIG. 14 is greater thanfor resonators according to FIG. 15, the passband edge allocated to thebranch having the resonators with the acoustic mirror can be formed moresteeply. If for example resonators R1 are realized with an acousticmirror, and resonators R2 are constructed as bridge resonators, asteeper left edge is obtained in the passband response of the reactancefilter constructed in this way.

Although the present invention has been presented and explained only onthe basis of a few exemplary embodiments, it is of course not limited tothese. Possible constructions of the present invention relate toadditional methods for varying the bandwidth of an individual resonator,and correspondingly to the use of resonators having different bandwidthsin inventive filters. The variations can thereby comprise individualresonators in one branch, individual resonators in both branches, allresonators in one branch, or all resonators in both branches.

1. A reactance filter comprising at least one basic element having afirst resonator in a first branch and a second resonator in a secondbranch, one of the branches being a serial branch and the other branchbeing a parallel branch, each resonator being of a BAW type having aspecific ratio V_(C)=C1/C0 of dynamic to static capacitance and theratio V_(C) for the resonator of the second branch being set smallerthan the ratio V_(C) for the resonator of the first branch.
 2. Areactance filter according to claim 1, wherein the resonator of thefirst branch is made of a first piezoelectric material and the resonatorof the second branch is made of a second piezoelectric materialdiffering therefrom, each piezoelectric material having a couplingcoefficient with the coupling coefficient of the first piezoelectricmaterial being higher than the coupling coefficient of the secondpiezoelectric material.
 3. A reactance filter according to claim 1,wherein the electrode materials for the resonators of the first andsecond branch are different, with the electrode material for theresonators of the first branch producing a higher effective couplingthan the electrode material of the resonators of the second branch.
 4. Areactance filter according to claim 1, in which the BAW resonators ofthe second branch include, besides a layer of a piezoelectric material,another layer of an additional material that has a lower dielectricconstant than the piezoelectric material between the two electrodes. 5.A reactance filter according to claim 1, wherein at least the resonatorsof the second branch include an acoustic mirror underneath an electrodelayer and wherein the effective coupling coefficient for the resonatorsof the second branch is lowered in relation to the coupling coefficientof the resonators of the first branch.
 6. A reactance filter accordingto claim 5, wherein the resonators of the first and second branch haveacoustic mirrors that are different with respect to one of the layerthicknesses of the mirror layers, the reflection characteristics in thetwo branches and the layer thickness of the mirror layer and reflectioncharacteristics.
 7. A reactance filter according to claim 5, whereinonly the resonators of the second branch have an acoustic mirror, andthe resonators of the first branch have a different method for thereflection of acoustic waves.
 8. A reactance filter according to claim1, wherein each branch has a plurality of basic elements wired with oneanother, with the basic elements of the serial branch being connected toone another in series, and the elements of the parallel branch beingconnected in parallel.
 9. A reactance filter according to claim 8,wherein the ratio V_(C)=C1/C0 of dynamic to static capacitance in atleast one resonator of the serial branch is set to a different valuethan the corresponding ratio of the resonators in the parallel branches.10. A reactance filter according to claim 1, wherein to obtain apassband response having a passband with a steeper left edge, the ratioV_(C)=C1/C0 of dynamic to static capacitance is lowered in at least oneresonator of the parallel branches in relation to the ratio for theresonators of the serial branch.
 11. A reactance filter according toclaim 1, wherein to obtain a passband response having a passband with asteeper right edge, the ratio V_(C)=C1/C0 of dynamic to staticcapacitance in at least one resonator of the serial branch is lowered inrelation to the ratio for the resonators of the parallel branches.
 12. Areactance filter according to claim 1, wherein the resonators of theparallel branches are connected in series with an inductance, and arerespectively connected individually with a ground terminal.
 13. Awireless communication system having a transmit part and a receiverpart, each part having a reactance filter comprising at least one basicelement having a first resonator in a first branch and a secondresonator in a second branch, with one of the branches being a serialbranch and the other branch being a parallel branch, each resonatorbeing a BAW type having a specific ratio V_(C)=C1/C0 of a dynamic tostatic capacitance and the ratio V_(C) for the resonator of the secondbranch being set smaller than the ratio of the ratio V_(C) for theresonator of the first branch, the filter for the transmit part havingthe second branch being a serial branch and the first branch being theparallel branch, so that the filter has a steeper right edge, and thefilter for the receive part having the second branch being the parallelbranch and the first branch being the serial branch, so that the filterhas a steeper left edge.
 14. A duplexer having two passband filters,each passband filter being a reactance filter comprising at least onebasic element having a first resonator in a first branch and a secondresonator in a second branch, one of the branches being a serial branchand the other branch being a parallel branch, each resonator being of aBAW type having a specific ratio V_(C)═C1/C0 of a dynamic to staticcapacitance and the ratio V_(C) for the resonator of the second branchbeing smaller than the ratio V_(C) for the resonator of the firstbranch, one of the passband filters having a low center frequency andbeing a reactance filter, with the second branch being a serial branchand the first branch being a parallel branch, so that it has a steeperright edge and the other of the two passband filters having a highcenter frequency and being a reactive filter with the second branchbeing a parallel branch and the first branch being a serial branch, sothat the other filter has a steeper left edge.